On a typical modern 64-bit Linux for example they're 40 bytes ie they are 320 bits. So yeah, unnecessarily bulky.

On my Linux system today Rust's Mutex<Option<CompactString>> is smaller than the pthread mutex type whether it is locked and has the text "pthread_mutex_t is awful" inside it or maybe unlocked with explicitly no text (not an empty string), either would only take like 30-odd bytes, the pthread_mutex_t is 40 bytes.

On Windows the discrepancy is even bigger, their OS native mutex type is this sprawling 80 byte monster while their Mutex<Option<CompactString> is I believe slightly smaller than on Linux even though it has the same features.